Examples of typical known art for obtaining molded articles from thermoplastic resin will be described below.
First, a cavity is preparatorily expanded to prevent the generation of weld marks on the surface of molded articles, and thermoplastic resin in a molten state, which is measured previously to a quantity required for obtaining the molded articles, is injected into the expanded cavity.
After this injection operation is completed and a gate is sealed, the thermoplastic resin injected into the cavity is cooled to a take-out temperature with a mold clamping force applied, and a molded article is obtained.
However, the method in the prior art involves the following problems.
In the case when one mold has two or more cavities, to completely equalize the ease of resin flow into the cavities is very difficult. Accordingly, much resin is injected into the cavity into which resin flow is easier than the other, and an imbalance occurs in the quantity of resin filling the cavities.
In addition, since the cavity is expanded to prevent weld marks when injection of the measured resin is completed, resin flow stops, leaving a large unfilled portion in the cavity. As a result, flow marks tend to occur along the boundary line of the filled portion and the unfilled portion.
Furthermore, photoelastic distortion occurs at a point where the flow mark generates.
Another example of prior art which is known is as follows.
First, the cavity is preparatorily expanded to prevent the generation of weld marks on the surface of a molded article, and thermoplastic resin in a molten state is injected to completely fill the cavity with the thermoplastic resin.
Then, with the gate opened, the cavity is reduced in volume, and overchanged thermoplastic resin is caused to backflow from the cavity. When the cavity is reduced to a desired volume, the gate is sealed, and the thermoplastic resin is cooled and hardened under pressure.
However, the above-mentioned example of the prior art has a problem in that due to a large quantity of backflow of the thermoplastic resin, photoelastic distortion tends to occur around the inlet of the cavity.
In addition, both the former and latter examples have the following problems.
Though when the thermoplastic resin is cooled from the molten state of a high temperature and hardened, its dynamic modulus of elasticity varies and the resin becomes hard, the dynamic modulus of elasticity does not vary linearly throughout the temperature range during cooling. Rather, the dynamic modulus of elasticity abruptly varies at a certain temperature (glass transition point, Tg).
Because of this characteristic, if there are temperature differences from part to part when the resin temperature goes through the glass transition point while being cooled, there will be a portion where the resin is in the solidified state and a portion where the resin is in the molten state. In the conventional molding method, no special consideration is given to the temperature differences from part to part in the resin, and the process proceeds to the compression process. As a result, the portion in the solidified state tends to cause plastic deformation and the internal composition tends to become heterogeneous.